Chronic hepatitis C affects 4 million patients in the United States, and results in 10,000 deaths annually. Major clinical consequences of chronic liver disease are related to the effect of hepatic fibrosis in producing portal hypertension and in the progressive decline of the functioning hepatic mass. Currently, measuring clearance rates of substances primarily removed from circulation by the liver provides the most sensitive, non-intrusive and specific indicator of liver function.
In humans, the two primary bile acids synthesized by the liver are cholic acid and chenodeoxycholic acid, which are converted into secondary bile acids by intestinal bacteria. These bile acids are conjugated with glycine or taurine and secreted by the liver. Serum bile acid levels are determined by the balance between intestinal absorption and hepatic elimination of bile acid.
Cholic acid is an example of a model bile acid. Orally administered cholic acid is absorbed across the epithelial lining cells of the small intestine, bound to albumin in the portal blood, and transported to the liver via the portal vein. Approximately 80 to 85% of cholic acid is extracted from the portal blood in its first pass through the liver. Cholic acid that escapes hepatic extraction exits the liver via hepatic veins that drain into the vena cava back to the heart, and is delivered to the systemic circulation. The area under the curve (AUC) of peripheral venous concentration versus time after oral administration of cholic acid quantifies the fraction of cholic acid escaping hepatic extraction and defines “oral cholate clearance”.
Intravenously administered cholic acid, bound to albumin, distributes systemically and is delivered to the liver via both portal venous and hepatic arterial blood flow. The AUC of peripheral venous concentration versus time after intravenous administration of cholic acid is equivalent to 100% systemic delivery of cholic acid. The ratio of the AUCs of orally to intravenously administered cholic acid, corrected for administered doses, defines cholate shunt.
After uptake by the liver, cholic acid is efficiently conjugated to either glycine or taurine and secreted into bile. Physicochemically cholic acid is easily separated from other bile acids and bile acid or cholic acid conjugates, using chromatographic methods.
An NIH-sponsored Hepatitis C Antiviral Long-Term Treatment against Cirrhosis (HALT-C) Trial examined whether long-term use of antiviral therapy (maintenance treatment) would slow the progression of liver disease. In noncirrhotic patients who exhibited significant fibrosis, effective maintenance therapy was expected to slow or stop histological progression to cirrhosis as assessed by serial liver biopsies. However, tracking disease progression with biopsy carries risk of complication, possibly death. In addition, sampling error and variation of pathologic interpretation of liver biopsy limits the accuracy of histologic assessment and endpoints. The histologic endpoint is less reliable because advanced fibrosis already exists and changes in fibrosis related to treatment or disease progression cannot be detected. Thus, standard endpoints for effective response to maintenance therapy in cirrhotic patients are prevention of clinical decompensation (ascites, variceal hemorrhage, and encephalopathy) and stabilization of liver function as measured clinically by Childs-Turcotte-Pugh (CTP) score. However, clinical endpoints and CTP score are insensitive parameters of disease progression.
In one proposal, studies were designed to analyze disease progression in a unique subset of patients with chronic hepatitis C, those with fibrosis and early, compensated cirrhosis. These patients are characterized by absence of clinical findings and normal or nearly normal values for standard routine biochemical parameters including serum albumin and prothrombin time. Child-Turcotte-Pugh scores will range from 5 to 6. For this reason, this subgroup of patients may benefit from quantitative tests of liver function that might be more useful than standard biochemical measurements, and more sensitive than clinical endpoints for evaluating the degree and progression of hepatic dysfunction.
Because early intervention of liver dysfunction is critical, a need exists for the detection of early signs that predict the onset or progression of a condition. A number of critical needs could be met by effective and accurate tests of hepatic function.